In a digital wireless communication system of recent years, multiplexing by a CDMA (Code Division Multiple Access) method or modulation by an OFDM (Orthogonal Frequency Division Multiplexing) method is used. A multiple modulated wave generated in some sort of communication systems such as a communication system using a CDMA method or an OFDM method has a characteristic to include a wave with instantaneous electric power much larger than average electric power. Therefore, a power amplifier for transmission of the above-mentioned communication systems needs to maintain linearity up to very high output level. This is because, suppressing the spread of a transmission spectrum caused by nonlinear distortion reduces adjacent channel leakage power. Further, instantaneous electric power is expressed using a peak factor or a crest factor and so on.
However, there is a problem that a power amplifier having good linearity up to very high output level, that is, up to a large amplitude component, has a large circuit scale, is expensive, and also has large power consumption. Therefore, a power amplifier having good linearity in a small amplitude component, but having nonlinearity in a large amplitude component is generally well used. For a power amplifier having such nonlinearity, by making an output back-off as small as possible, further efficiency improvement can be implemented. The back-off is to lower average electric power to prevent saturation and so on. However, when the back-off is decreased, because an input signal tends to enter a nonlinearity region of an amplifier, adjacent channel leakage power which is a distortion component of a wireless transmission signal is apt to become large.
In order to suppress adjacent channel leakage power caused by nonlinear distortion of a power amplifier, various distortion compensation technologies are proposed. Among them, as a distortion compensation technology adopted most often in recent years, there is a digital pre-distortion method.
FIG. 5 is a block diagram showing an exemplary structure of a wireless transmitter including a distortion compensation circuit of a digital pre-distortion method. A wireless transmitter of FIG. 5 includes transmission data generation unit 21, D/A converter (Digital-Analog Converter. Hereinafter called as “DAC”) 22A and 22B, quadrature modulator 23, reference signal generator unit 24, power amplifier 25 and distortion compensation circuit 20. Distortion compensation circuit 20 includes nonlinear distortion compensation operation unit 4, directional coupler 26, quadrature demodulator 27 and A/D converter (Analog-Digital Converter. Hereinafter called as “ADC”) 28A and 28B, distortion compensation coefficient update unit 5A and electric power computation unit 8.
The nonlinear distortion compensation operation unit 4 performs distortion compensation operation for digital quadrature baseband signals I and Q from transmission data generation unit 21 by complex multiplication based on distortion compensation coefficients K and θ calculated in advance. Quadrature baseband signals I″ and Q″ after distortion compensation operation is performed, are converted by the DACs 22A and 22B into analog signals and become analog quadrature baseband signals. And the quadrature modulator 23 converts analog quadrature baseband signals into quadrature modulation signals using a signal from the reference signal generator unit 24. Quadrature modulation signals are power amplified by the power amplifier 25, and outputted as an RF (Radio Frequency) signal.
Part of the RF signals are fed back to the quadrature demodulator 27 via the directional coupler 26, and are demodulated to analog quadrature baseband signals using a signal from the reference signal generator unit 24. Analog quadrature baseband signals are converted by the ADCs 28A and 28B into digital signals and become feedback quadrature baseband signals Ib and Qb.
The distortion compensation coefficient update unit 5A compares feedback quadrature baseband signals Ib and Qb and quadrature baseband input signals I and Q from the transmission data generation unit 21 and calculates a distortion compensation coefficient. The distortion compensation coefficient update unit 5A updates the distortion compensation coefficient to the latest value and stores it. At this time, the distortion compensation coefficient update unit 5A stores the distortion compensation coefficient by correlating it to electric power of quadrature baseband signals I and Q. Electric power of quadrature baseband signals I and Q is calculated by the electric power computation unit 8.
The nonlinear distortion compensation operation unit 4 reads the distortion compensation coefficient correlated to electric power of quadrature baseband signals I and Q and performs distortion compensation using the read distortion compensation coefficient. By the above mentioned signal processing, adaptive distortion compensation is implemented.
The circuit configuration of a distortion compensation circuit of a digital pre-distortion method described above is one example. A circuit configuration which adopts digital method for quadrature modulation and quadrature demodulation or a circuit configuration using a frequency converter instead of direct modulation is also proposed. Also, a circuit configuration which performs distortion compensation operation using a distortion compensation coefficient correlated to amplitude instead of a distortion compensation coefficient correlated to electric power is also proposed. Further, amplitude is proportional to a square root of electric power. Accordingly, by setting up amplitude computation unit which computes amplitude, amplitude can be obtained based on electric power.
An example of an input/output characteristic of a wireless transmitter including a distortion compensation circuit of the digital pre-distortion method mentioned above is shown in FIG. 6. FIG. 6 indicates input/output characteristics of the nonlinear distortion compensation operation unit 4, the power amplifier 25 and an entire wireless transmitter respectively per input level normalized by setting saturated input level of the power amplifier 25 as 1. It can be found that, by giving an inverse characteristic of the input/output characteristic of the power amplifier 25 having nonlinearity in the nonlinear distortion compensation operation unit 4, a linear input/output characteristic is obtained for the entire wireless transmitter.
However, as the input/output characteristic of the nonlinear distortion compensation operation unit 4 indicates, when distortion compensation is made, at an input level of about 0.5-0.6, which is about 4 dB-6 dB lower than 1 which is saturated input level, signal level after distortion compensation operation reaches a saturated point of the power amplifier 25. In case of no compensation, a phenomenon such as a range of input level which can be inputted without making the power amplifier 25 being saturated is limited to a range with a minimum below the saturated input level does not occur normally.
And an amount for which input level is limited against saturated input level is equal to a decreased amount of an actual gain against a linear gain of a saturated output point. In other words, a limited amount of input level and a decreased amount of linear gain are equal. This means that, for a distortion compensation circuit of a digital pre-distortion method of FIG. 5, a relation of output back-off=input back-off holds. As mentioned above, for the distortion compensation circuit of a digital pre-distortion method of FIG. 5, in order to avoid the generation of clipping distortion in a saturated region of an amplifier, input level needs to be limited. Further, although occurrence of clipping distortion can be prevented by limiting input level, a distortion compensation circuit of a pre-distortion method cannot compensate clipping distortion which occurred once.
The limitations mentioned above relating to a distortion compensation circuit of a pre-distortion method also apply similarly to an instantaneous input. That is, by performing distortion compensation, an output of a power amplifier reaches a saturated region for instantaneous input level also, which is several dB lower than saturated input level of in case of no compensation. Therefore, there is a problem that clipping distortion increases and distortion compensation effect declines.
Further, “clipping distortion” is a ratio of leakage power at frequency deviated for certain frequency from center frequency, which is generated when a signal is inputted to an amplifier with an ideal amplifier characteristic having an ideal limiter characteristic, against electric power at center frequency. Further, “ideal limiter characteristic” is a characteristic by which an amplitude-amplitude characteristic (hereinafter called as “AM (Amplitude Modification)/AM characteristic”) is linear up to a saturated point. “Ideal amplifier characteristic” is a characteristic which has “ideal limiter characteristic” and whose amplitude-phase characteristic (hereinafter called as “AM/PM (Phase Modification) characteristic” is flat.
Thus, distortion compensation by a digital pre-distortion method compensates only nonlinear distortion of AM/AM characteristics and AM/PM characteristics generated by a power amplifier. Distortion compensation by a digital pre-distortion method cannot compensate clipping distortion in a saturated region. As mentioned above, a multiplex modulated wave which is generated in a communication system using a CDMA method or in a communication system using OFDM modulation and so on includes a wave with instantaneous power whose amplitude is very large against average electric power. Accordingly, as clipping distortion is apt to occur, it is important to limit instantaneous maximum electric power of an input signal not to exceed the saturated electric power of a power amplifier for transmission.
Further, distortion compensation equipment by a pre-distortion method is disclosed, for example, in Japanese Patent Application Laid-Open No. 2002-223171. A method to suppress amplitude adaptively is disclosed, for example, in Japanese Patent Application Laid-Open No. 2004-064711.
A distortion compensation circuit for suppressing very large instantaneous amplitude which occurs on an envelope, and which becomes a factor of the above-mentioned problem, is disclosed in Japanese Patent Application Laid-Open No. 2001-251148 (hereinafter referred to as “patent document 1”) and Japanese Patent Application Laid-Open No. 2003-168931 (hereinafter referred to as “patent document 2”).
In the distortion compensation circuit disclosed in patent document 1, the problem mentioned above is resolved by setting limitation to a distortion compensation coefficient calculated in the distortion compensation coefficient update unit 5A of FIG. 5. FIG. 7 is a block diagram showing an exemplary structure of a distortion compensation coefficient update unit disclosed in patent document 1.
As shown in FIG. 7, the distortion compensation coefficient update unit 5B includes distortion compensation coefficient data memory 7, distortion compensation coefficient calculation unit 6, limit value setting unit 16, distortion compensation coefficient correction portion 12 and coefficient limitation determination unit 13. Further, in FIG. 7, an identical code is attached to a component identical to a component shown in FIG. 5. Also, in FIG. 7, part of the names of components in the drawings disclosed in patent document 1 are modified or simplified.
The distortion compensation coefficient calculation unit 6 compares quadrature baseband signals I and Q with feedback quadrature baseband signal Ib and Qb on the polar coordinates, calculates an amplitude difference and a phase difference, and calculates distortion compensation coefficient h based on them. The coefficient limitation determination unit 13 computes, using distortion compensation coefficient h calculated in the distortion compensation coefficient calculation unit 6, electric power x of a signal after distortion compensation operation to inputted quadrature baseband signals I and Q is performed. By comparing electric power x and upper limit electric power Pmax set to the limit value setting unit 16 in advance, it is judged whether limitation is given to a distortion compensation coefficient. And determination result and calculated electric power x after distortion compensation operation are outputted to the distortion compensation coefficient correction unit 12.
In case electric power x after distortion compensation operation is no more than upper limit electric power Pmax, limitation is not given to distortion compensation coefficient h. In this case, the distortion compensation coefficient correction unit 12 outputs distortion compensation coefficient h calculated in the distortion compensation coefficient calculation unit 6 to the distortion compensation coefficient data memory 7 just as it is.
On the other hand, when electric power x after distortion compensation operation exceeds upper limit electric power Pmax, limitation is given to distortion compensation coefficient h. In this case, the distortion compensation coefficient correction unit 12, after making a correction that distortion compensation coefficient h calculated in the distortion compensation coefficient calculation unit 6 becomes 1/m times as much, outputs it to the distortion compensation coefficient data memory 7. Here, m is an amplitude limitation coefficient, and is calculated using the following formula:m=(x/Pmax)1/2.
The distortion compensation coefficient data memory 7 stores the distortion compensation coefficient h by correlating it to electric power of quadrature baseband signals I and Q from the electric power computation unit 8.
Here, by using a distortion compensation circuit including the distortion compensation coefficient update unit 5B as shown in FIG. 7, it is possible to correct the size of an amplitude compensation coefficient while to maintaining the phase to an input signal, and to limit signal amplitude after distortion compensation operation.
In the technology described in patent document 1, because limitation is being made to the amplitude compensation coefficient itself, amplitude of feedback quadrature baseband signals Ib and Qb from the power amplifier 25 (refer to FIG. 5) is limited. However, amplitude of quadrature baseband signals I and Q is not limited. That is, in the distortion compensation coefficient calculation unit 6, a distortion compensation coefficient is calculated by comparison operation between quadrature baseband input signals I and Q for which amplitude is not limited and feedback quadrature baseband signals Ib and Qb for which amplitude is limited. Therefore, an amplitude difference and a phase difference between feedback quadrature baseband signals Ib and Qb, and I and Q cannot be obtained correctly. Accordingly, there is a problem that highly precise distortion compensation cannot be implemented.
As a method of resolution for this problem, there is a method disclosed in patent document 2. The method of patent document 2 utilizes the fact that, when input amplitude does not exceed the distortion compensable upper limit input amplitude (in an example of FIG. 6, about 0.5-0.6) of the nonlinear distortion compensation operation unit 4, amplitude after distortion compensation operation does not exceed saturated input amplitude of a power amplifier. In other words, the method disclosed in patent document 2 is a method to limit a maximum value of amplitude based on the relation of output back-off=input back-off. That is, in a preceding step of a circuit in which distortion compensation operation is made, a maximum value of amplitude is limited to the level in which input amplitude will be the amplitude where output back-off is added to an effective value.
FIG. 8 is a block diagram showing one exemplary structure of a wireless transmitter described in patent document 2. Further, in FIG. 8, an identical code is attached to a component identical to the component shown in FIG. 5. Distortion compensation circuit 30 of FIG. 8 includes electric power computation unit 1, coefficient calculation unit 14, limit value setting unit 15 and amplitude limitation unit 3 in the receding step of the nonlinear distortion compensation operation unit 4, and other components are same as FIG. 5. Further, in FIG. 8, part of the names of components in the drawings disclosed in patent document 2 are simplified or modified.
The electric power computation unit 1 computes electric power P of quadrature baseband signals I and Q and outputs electric power P to the coefficient calculation unit 14. The coefficient calculation unit 14 compares electric power P and electric power limit value Pth set to the limit value setting unit 15 in advance. The coefficient calculation unit 14 decides not to perform amplitude limitation in case the electric power P is no more than electric power limit value Pth, and outputs 1 as a multiplier coefficient. On the other hand, in case the electric power P is larger than the electric power limit value Pth, the coefficient calculation unit 14 decides to perform amplitude limitation, and output (Pth/P)1/2 as a multiplier coefficient. Further, a multiplier coefficient when amplitude limitation is performed is not limited to this value, and any value such that electric power value P of quadrature baseband signals I and Q become not more than limit value Pth will do.
The amplitude limitation unit 3 performs amplitude limitation by “circular clipping” by multiplying a multiplier coefficient from the coefficient calculation unit 14 to each of an I component and a Q component of a quadrature baseband signal from transmission data generation unit 21. The “circular clipping” is a clipping where the amplitude which is a result of an I component and a Q component being synthesized is limited to no more than a fixed size for all phases. The nonlinear distortion compensation operation unit 4 performs distortion compensation by complex multiplication based on a distortion compensation coefficient, to quadrature baseband signals I′ and Q′ after amplitude limitation for the signals is performed. An updating method of a distortion compensation coefficient and a referring method of a distortion compensation coefficient are similar to an example mentioned above.
Thus, in the distortion compensation circuit described in patent document 2, nonlinear distortion compensation is performed, by a circular clipping on the orthogonal coordinates, after performing amplitude limitation of a quadrature baseband transmission signal. Accordingly, the distortion compensation circuit described in patent document 2 will not err in operations to obtain an amplitude difference and a phase difference, suppress clipping distortion component and can improve the later nonlinear distortion compensation effect substantially.